1. Field of the Invention
The present invention relates to a light-scanning device employed in complex machines such as a copying machine, a facsimile, a printer, etc. and an image-forming apparatus in which the light-scanning device is installed.
2. Description of Related Art
In an image-forming apparatus which employs an electrophotographic process known as the Carlson process, as a photoreceptor drum which is an example of an image carrier rotates, a toner image is formed on a surface of the photoreceptor drum through sequential processes of uniform electrification of the surface of the photoreceptor drum, latent image formation by light exposure, developing, transferring and fixing. In the processes, the so-called light-scanning device is used to form the latent image on the photoreceptor drum.
A general structure of the optical scanning device is explained with reference to FIG. 9. In the optical scanning device, laser light emitted from a semiconductor laser 101 is coupled by a coupling lens 105. The laser light passes through an aperture (not shown) to adjust the laser light and enters a cylindrical lens 109 being powerful only in a sub-scanning direction. The laser light emitted from the cylindrical lens 109 is imaged as a long linear image in a main scanning direction in a vicinity of a surface of a polygon mirror 111 of a polygon scanner 100 and is subjected to deflection scanning in the main scanning direction by the polygon scanner 100. The laser light which is subjected to the deflection scanning is focused via a first scanning lens 113 and a second scanning lens 121 to scan the surface of the image carrier (not shown) at an approximately constant speed.
As shown in FIG. 10, the polygon scanner 100 is composed of the rotatable polygon mirror 111, a motor 4 to rotate the polygon mirror 111 and a driving control circuit board 3. The motor 4 is supported by a motor bearing 5. A bearing accommodation section 2 housing the motor bearing 5 is integrated with the driving control circuit board 3 by caulking or the like. The rotatable polygon mirror 111 is fixed on a rotation shaft 6 by a fixation spring 7. The rotatable polygon mirror 111 is rotated by the rotation of the rotation shaft 6. An integrated circuit 12 for controlling the motor 4, etc. is installed on the driving control circuit board 3.
Such a polygon scanner 100 is fixedly positioned by a plurality of screws 8 in an optical housing 9. The optical housing 9 is usually made of a resin. The optical housing has at an upper portion thereof an opening 9a, which is covered by a cover 13 made of a thin metallic plate or synthetic-resinous plate, etc. A gasket (not shown) to seal up an inside of the optical housing 9 may be provided between the optical housing 9 and the cover 13.
Meanwhile, in response to the demand for an image of high-density and for high-speed printing, the rotational speed of the rotatable polygon mirror 111 has been raised up to tens of thousands rpm, which causes the following two problems. One of the problems is the noise increment caused by the wind noise. The other is the contamination of the reflecting surfaces of the rotatable polygon mirror caused by dust contained in the air.
The first problem is explained as follows. When the rotatable polygon mirror is rotated, the surrounding air is also rotated. The flow velocity of the airstream achieves its maximum at the periphery of reflecting surfaces 111b of the rotatable polygon mirror, which is the largest part in the composition of the polygon scanner. Because of this, an airstream is generated as shown by the broad arrows in FIG. 11. Because the rotatable polygon mirror has a polyhedral-shape, the airstream is obstructed by an angle of the adjoining portion of the adjacent reflecting surfaces 111b, to generate the wind noise. The frequency and the level of the wind noise rise depending on the increase in the rotational speed of the rotatable polygon mirror 111. This wind noise constitutes the majority of the noise of a conventional polygon scanner.
With respect to the second problem, the dust contained in the air enters into the optical housing 9 from the outside thereof and is drawn into the high-speed airstream along with the rotation of the rotatable polygon mirror 111 at high speed. This dust circulates in the vicinity of the reflecting surfaces 111b of the rotatable polygon mirror and is adhered on the reflecting surfaces of the rotatable polygon mirror, as shown in FIG. 12, which causes contamination of the reflecting surfaces 111b. Consequently, the reflectivity is significantly decreased with the reduction in light quantity and light quantity irregularity occurs. In particular, this phenomenon tends to be more notable with the lapse of time.
The following two broadly divided countermeasures can be considered as solutions to the above-mentioned two problems (noise and dust). One countermeasure is to seal the polygon scanner, whereby the wind noise can be insulated from outside, so as to protect the reflection side of the polygon mirror from the dust in the air. Another countermeasure is not to seal the polygon scanner, and by providing a member for obstructing the air current generated by the rotation of the rotatable polygon mirror, the air current never hits the polygon mirror directly. Technologies for sealing a polygon scanner are disclosed, for example, in Japanese Patent Number 2774444 and Japanese Application Publication Number Hei11-264950.
When sealing the polygon mirror, the problems, as indicated in Japanese Patent Number 2774444 and Japanese Application Publication Number Hei11-264950, will be explained with reference to FIG. 13. There are provided with a sidewall 21 standing in the optical housing 9; a transmission optical element 22 forming part of the sidewall whereby the deflected laser beam is transmitted to the outside of the sidewall at the rotatable polygon mirror 111; and a plate-like cover 23 fixed at the top edge of the sidewall thereby separating the space formed by the sidewalls for seclusion. The sealed polygon chamber 10 is formed by surrounding the polygon scanner 100.
When the polygon chamber 10 is sealed, there may arise the following problems. One of the problems is that the calorific value of the integrated circuit caused by the electric current that flows in an integrated circuit 12 on a driving circuit control substrate 3 of the polygon scanner 111 increases because the polygon chamber 10 has been sealed. Another problem lies in the act that when the calorific value from the rotational driving part such as the motor 4 increases, the temperature of the inside of the sealed up polygon chamber 10 rises remarkably. Due to the temperature rise, the service lives of both the integrated circuit 12 and the motor bearing 5 (rotational driving part) are shortened and the evaporated film of the reflecting surfaces of the rotatable polygon mirror 111 is oxidized and peels off.
In addition, there occurs an abnormality in the image resulting from the deviation of the image-forming position caused by a different reflection direction of the laser beam because when the temperature rises, the bottom of the housing in the polygon chamber 10 transforms and the rotation shaft 6 of the rotatable polygon mirror 111 tilts (the tilt of the shaft).
In addition, when the heat generated in the polygon chamber 10 is transmitted to the outside thereof, for the situations wherein the optical housing itself for positioning and fixing the various members in the optical housing deforms or the heat remains in the optical housing and thus generates a temperature gradient, for instance, if the image-forming means (the scanning lens) is made of plastic, because the expansion of the scanning lens is not uniform, the beam spot diameter of the photoreceptor becomes abnormal and the beam spot position varies with the lapse of time. Such an abnormality in the beam spot diameter and the change in the beam spot position are the major causes of the occurrence of picture abnormality such as the so-called “color shift” in the final printing to form color pictures or density irregularities of monochromic pictures.
To solve the problem posed by the temperature rise, the following technologies for lowering the temperature have been disclosed. For instance, in Japanese Application Publication Number Hei10-123447, a heat-radiating element was installed; in Japanese Application Publication Number 2002-214552, a cooling element was installed. Moreover, in Japanese Application Publication Number Hei10-221633, an air pressure was decompressed in a sealed polygon chamber and in Japanese Utility-Model Application Publication Number S 62-104218, a heat-generating part was joined to the cap with a heat pipe for radiating heat.
However, since these inventions enlarge the optical housing and make the light-scanning device complex, which is related to the increase in the dimensions and cost of the entire image-forming apparatus, the marketability of the image-forming apparatus is undermined notably.
Meanwhile, Japanese Application Publication Number 2002-267990 discloses in order to provide a polygon scanner motor which is capable of abating the wind noise to obtain low noise. The element characterized in this literature is that, instead of sealing the optical housing, a circular washer for abating the wind noise is fitted above (or above and below) the rotatable polygon mirror. A downward (and an upward) airstream generated by the rotation of the rotatable polygon mirror is obstructed by the circular washer and never directly hits the rotatable polygon mirror. Thus, the problems caused by the temperature rise in the polygon chamber are solved.
However, since the circular washer rotates together with the rotatable polygon mirror at high speed, it is very time-consuming to balance the polygon scanner as the body of rotation after the assembly of the polygon scanner, which may lead to a cost increase consequently. The polygon scanner is also prone to the balance disruption with the lapse of time. When the balance disruption with the lapse of time occurs, it is necessary to replace the polygon scanner or the light-scanning device, which is not in keeping with the common practice of resource saving.